Post filter processing is a filter processing for an image after some processing is executed for the image. For example, after a dynamic image is decoded, post filter processing reduces noise caused by compression in case of coding the dynamic image. Especially, in image coding methods using DCT (Discrete Cosine Transformation) such as MPEG method, block noise is generated because of coding by unit of block, and mosquito noise as quasi-high frequency element is generated because of quantization. Furthermore, a high frequency element is lost by quantization in case of coding. As a result, an edge part in an image becomes dull and the image blurs.
In order to improve these defects, in general, the post filter processing is adaptively executed based on characteristics of the image. However, in typical post filter processing, the filter processing is executed pixel-by-pixel on the image, and a large calculation quantity is necessary. For example, in case of CIF format used for “ITU-TH.261, H.263, ISO/IEC MPEG-4”, as for luminance signal on the image, “352” pixels along a horizontal line and “288” pixels along a vertical line are included, and the filter processing must be executed for a total of “101, 376” pixels. If such processing of large calculation quantity is performed by software, the real time processing becomes difficult. If it is performed by hardware, the cost becomes expensive.
On the other hand, in case of size conversion of coded image, a size conversion method using a filter after decoding and a size conversion method using conversion coefficients such as DCT coefficient, are well known. In both methods, if the size of an image including a coded noise is converted, the coded noise is also expanded by the size conversion processing. Accordingly, even if the post filter processing is executed after size conversion, the coded noise remains in the image. Furthermore, if the size conversion processing is executed after the post filter processing, in case of magnification, the image becomes blurred. In case of reduction, an aliasing is generated by sharpening processing.
As mentioned-above, in the post filter processing of the prior art, the filter processing is executed pixel-by-pixel on the image. Accordingly, a large calculation quantity is necessary. Furthermore, in the size conversion processing of the image, in case of size conversion of coded image, the post filter processing cannot be effectively executed for the encoded image.